HF links notably offer a capacity outside of the line of sight or BLOS which makes it possible to implement long or even very long haul communications without requiring the use of a satellite.
The technical context of the present invention relates, more particularly, to the use of a plurality of HF channels (so-called multi-carrier usage) as proposed in the standard MIL STD 188-110B Annex F known to a person skilled in the art, in which is described the use of two 3 KHz or general-purpose carriers described in the patent application filed by the Applicant under the No FR 10/04650 which proposes considering the use of a plurality n of conventional channels or paths with a typical width of 3 KHz bandwidth or more, that can also be channels such as those proposed in the standard MIL STD 110-188C Annex D, with bandwidths of 3, 6, 9, 12, 15, 18, 21 or 24 kHz.
The long-haul communication capability (in BLOS mode) of the HF links relies on the reflection of the HF waves (which range typically from 2 to 30 MHz) on the layers of the ionosphere, layers whose qualities are not stable in time and in space, which leads to strong propagation channel variations. To this channel instability is also added the ever-present possibility of different interfering factors, intentional or otherwise, in particular at night when the usable part of the HF spectrum is less great.
Despite its instability, the channel offers the interest of allowing for long-haul communications without it being necessary to first deploy a complicated or costly infrastructure, unlike satellite communications for example. When its better stealth is also considered, this explains why the professionals are seeking to increase the bit rates offered on the HF links. A solution has been proposed in the abovementioned patent application for considering the use of a plurality n of 3 KHz channels, contiguous or not, in order to achieve higher useful bit rates (above 20 kb/s) for the users of the HF band.
According to Shannon's theorem, the bit rate can be augmented either by augmenting the spectral efficiency or by augmenting the band used. At the present time with the HF systems already operating with high spectral efficiency, the decisive gain is expected of bandwidth augmentation strategies. Two main approaches can be envisaged:                a widening of the standard channel structure, as proposed in the standard MIL STD 188-110 revision C which provides for the use of channels up to 24 KHz bandwidth,        the use of a plurality of standard channels, typically 3 KHz, concatenated in order to constitute a sufficient bandwidth, as proposed in the Applicant's two patent applications FR 10 04560 and FR 11 03083. This approach is compatible with the use of widened channels, since such channels can also be concatenated.        
The first approach raises the problem of the availability of the bandwidth, and the lower efficiency in the case of operation on a partially occupied band.
The second approach, which makes it possible to select channels of good quality, raises the problem of the peak factor, that is to say the variation of the amplitude of the signal, between its maximum value (peak) and its average value. Also traditionally referred to is the ratio between the peak value and the average value (or Peak-to-Average Power Ratio, PAPR) for operating correctly with the power amplifier.
One technical problem that is posed is therefore as follows: how to limit the peak factor of a multi-carrier wave form to allow for an effective use of unconnected bands for one and the same HF or other wide band link and therefore to allow for the use of multi-carrier wave forms that are compatible with the current spectral allocations in the HF band or for other frequencies.
The state of the art on the issue of reducing the peak factor, as known to the Applicant, proposes different techniques cited hereinbelow.
A first technique is to use single-carrier solutions having a lower back-off factor than multi-carrier solutions. These single-carrier-based solutions prove complex to equalize in high bandwidth cases. Typically, it is currently difficult to produce an equalizer on an ionospheric HF channel with good performance levels beyond a useful bandwidth of 12 KHz. They also pose the problem of the availability of the bandwidth concerned, the use of a single-carrier requiring a contiguous allocation, whereas, in multi-carrier cases, it is possible to envisage the use of a set of non-contiguous sub-bands, as is described in the abovementioned patent application FR 10 04560.
Another technique is the use of modulations with low peak factors, of continuous phase modulation, or CPM, type. These solutions pose the problem of their significant spectral width, high shoulders, and therefore their non-conformity to the standard radio masks, save by requiring very high spectral occupancies, which are not envisaged currently in HF mode.
Another technique is the use of clipping solutions. The main fault with these techniques is that they degrade the signal. With the signal having been degraded by the clipping and filtering operations, the performance levels are naturally degraded, this degradation generally being corrected by the function-adapted correcting coding in conjunction with a cost in terms of transmitted useful bit rate.
The use of coding-based solutions makes it possible to effectively reduce the power dynamic of the signal. However, the major drawback with these solutions is the reduced spectral efficiency naturally induced by the coding techniques.
The signal addition-based solutions are effective for reducing the power dynamic of the signal. Numerous solutions exist in this field, their main defect being that they degrade the energy yield of the transmitter because the added signal does not convey any useful information.
Another technique is to use constellation extension-based solutions. These solutions by definition modify the constellation to be transmitted, thus modifying the Euclidian distance between the points of the reference constellation. Consequently, the performance levels are naturally degraded.
Other, more basic solutions, such as reducing the amplification power or working with modulations with a lower peak factor, such as phase shift keying (PSK) instead of quadrature amplitude modulation (QAM) involve greatly reducing the useful bit rate that can be offered and are therefore not retained.
The prior art solutions do not therefore make it possible to obtain a reduction of the peak factor without degrading the signal, using a greater bandwidth or modifying the transmitted signal.